c# - 刚体圆周运动

Question

我有一个 2D 球，它有一个力并在不确定的方向上移动，我需要让球在用户触摸屏幕时相对于当前方向向右或向左圆形移动，而不改变速度。我该怎么做？

我试过这段代码，但是其他物理不能影响球，因为我们直接改变变换:

float angle = 0;
float radius = 1;

void FixedUpdate()
{
    angle += speed * Time.deltaTime;
    float x = center.x + Mathf.Cos(angle) * radius;
    float y = center.y + Mathf.Sin(angle) * radius;

    rigidbody.transform.position = new Vector3(x, y, 3);
}

Answer 1

就像物理学做的那样:

你的触摸位置有点像一个重力中心 → 使用相同的幅度连续分配一个新方向，但在一个新方向上，该方向与从中心到移动物体的矢量和旋转轴成 90° 角。

在代码中这可能看起来像

public class Example : MonoBehaviour
{
    [SerializeField] private Rigidbody2D rb;
    [SerializeField] private Camera _camera;

    [SerializeField] private Vector2 initialVelocity;    

    private bool doGravitation;
    private Vector2 centerPos;

    private void Awake()
    {
        if(!rb) rb = GetComponent<Rigidbody2D>();
        if(!_camera) _camera = Camera.main;

        rb.velocity = initialVelocity;
    }

    private void Update()
    {
        // check for touch support otherwise use mouse as fallback
        // (for debugging on PC)
        if (Input.touchSupported)
        {
            // not touching or too many -> do nothing
            if (Input.touchCount != 1)
            {
                doGravitation = false;
                return;
            }

            var touchPosition = Input.GetTouch(0).position;
            centerPos = _camera.ScreenToWorldPoint(touchPosition);
            doGravitation = true;
        }
        else
        {
            // mouse not pressed -> do nothing
            if (!Input.GetMouseButton(0))
            {
                doGravitation = false;
                return;
            }

            centerPos = _camera.ScreenToWorldPoint(Input.mousePosition);
            doGravitation = true;
        }
    }

    private void FixedUpdate()
    {
        if(!doGravitation) return;

        // get current magnitude
        var magnitude = rb.velocity.magnitude;

        // get vector center <- obj
        var gravityVector = centerPos - rb.position;

        // check whether left or right of target
        var left = Vector2.SignedAngle(rb.velocity, gravityVector) > 0;

        // get new vector which is 90° on gravityDirection 
        // and world Z (since 2D game)
        // normalize so it has magnitude = 1
        var newDirection = Vector3.Cross(gravityVector, Vector3.forward).normalized;

        // invert the newDirection in case user is touching right of movement direction
        if (!left) newDirection *= -1;

        // set new direction but keep speed(previously stored magnitude)
        rb.velocity = newDirection * magnitude;
    }
}

另见:

• Input.GetTouch
• Touch.position
• Camera.ScreenToWorldPoint
• Vector2.SignedAngle
• Vector3.Cross

---

请注意，这有时看起来很奇怪，尤其是在球前面接触时，因为我们将其强制到可能与当前运动具有奇怪角度的圆形曲线上。你可以解决这个问题，但这取决于你；)